A projection display apparatus (projector) which produces an image through modulating light from a light source by a spatial light modulator to project the modulated light on a screen through a projection lens is conventionally known. As the spatial light modulator, for example, a liquid crystal display (LCD) panel, a digital micromirror device (DMD) or the like is used. Types of the spatial light modulator include a transmissive one which transmits modulated incident light, and a reflective one which reflects modulated incident light.
A projector using the reflective spatial light modulator forms an image using a phenomenon in which among polarized components selected (transmitted or reflected) by a polarization selecting device such as a PBS (polarizing beam splitter) at the time of the entry of light, a component of which the polarization state is changed by the spatial light modulator is reversely selected (reflected or transmitted) by the polarization selecting device so as to be traveled in a direction different from the light source.
A part controlling light modulation in the reflective projector will be described in detail below referring to FIG. 17. In FIG. 17, a numeral 100 represents an optical axis. As shown in the drawing, only S-polarized component light 200S in incident light 200 from the light source is selected (reflected) on a polarization selecting surface 101A of a PBS 101 to reach a reflective liquid crystal display panel 102 as a spatial light modulator. In a state that the reflective liquid crystal display panel 102 has no influence on the polarization state of the reached light (off state), the reached light which remains S-polarized is reflected on the reflective liquid crystal display panel 102 to return to the PBS 101, and the S-polarized light as S-polarized component light 201S is reflected on the polarization selecting surface 101A of the PBS 101 in a direction opposite to a direction where light enters to return to the light source side.
On the other hand, in a state that the reflective liquid crystal display panel 102 has an influence on the polarization state (on state), a part or all of reflected light from the reflective liquid crystal display panel 102 is converted into P-polarized component light 201P to pass through the polarization selecting surface 101A of the PBS 101. The passed P-polarized component light 201P forms an image on the screen through a projection lens (not shown). Gray scale is controlled by the amount of change in the polarization state in the reflective liquid crystal display panel 102.
Alternatively, contrary to the state shown in FIG. 17, the incident light from the light source enters from a front side of the reflective liquid crystal display panel 102, and a light beam selected by reflection on the polarization selecting surface 101A of the PBS 101 in light returned from the reflective liquid crystal display panel 102 can be guided to the projection lens.
In such a reflective projector, in the off state, all light beams which are S-polarized component light 200S when the light beams enter the reflective liquid crystal display panel 102 must be returned to the light source side as the S-polarized component light 201S after the reflection (when the light beams emit). However, in reality, some of the light beams are converted into P-polarized component light 201P to pass through the PBS 101.
The reason will be described referring to FIGS. 18 and 19 below. FIGS. 18 and 19 show an optical positional relationship of the polarization selecting surface 101A of the PBS 101 at the time of the enter of light and at the time of the emission of light.
An electric field direction of each of P-polarized light and S-polarized light is determined by a traveling direction of a light beam and a direction of a normal n1 in a plane of incidence (polarization selecting surface 101A). Therefore, as shown in FIG. 18, when the polarization selecting surface 101A before the entry of light is parallel to that after the entry of light, that is, the direction of the normal is the same before and after the entry of light, the directions of the P-polarized light and the S-polarized light before the entry of light and after the emission of light coincide with each other. In such an ideal state, light 200S reflected on the PBS 101 as the S-polarized component on an incident side becomes S-polarized component light 201S on an emission side.
However, in an optical system in a practical projector as shown in FIG. 17, the above-described ideal positional relationship between the light beam and the PBS 101 is not established. In a practical optical system, a light beam is reflected on the reflective liquid crystal display panel 102, so as shown in FIG. 19, a relationship of the polarization selecting surface 101A at the time of the entry and at the time of the emission is a symmetric (mirror) relationship with respect to a plane including the reflective liquid crystal display panel 102. Therefore, the direction of the normal of the polarization selecting surface 101A at the time of the entry is different from that at the time of emission, thereby the electric field directions of the P-polarized light and the S-polarized light are not the same, and the light 200S reflected as the S-polarized component at the time of the entry includes the P-polarized component light 201P on the polarization selecting surface 101A at the time of the emission even in the off state that the reflective liquid crystal display panel 102 has no influence on the polarization state. The P-polarized component is not removed at the time of the emission and reaches an image area which is supposed to be black, thereby image quality (mainly a extinction ratio) is impaired.
As described above, there is a problem that in the case where two or more surfaces where light enters exist, unless the surfaces are parallel to each other, a general light beam includes a different polarized component on each surface, so a component which is supposed to be removed at the time of the emission remains, thereby image quality is impaired.
A typical solution to the problem is to place a ¼-wavelength retardation plate (quarter-wave plate) between the reflective liquid crystal display panel 102 and the PBS 101 to correct the polarization state. In this case, the light beam passes back and forth through the quarter-wave plate twice, so the quarter-wave plate effectively functions as a half-wave plate.
FIG. 20 shows an optical positional relationship of each optical device at the time of the entry of light and at the time of the emission of light in the case where the quarter-wave plate is placed. In FIG. 20, when an axis of a quarter-wave plate 121 is set to be vertical to a paper surface, a light beam pass back and forth through the quarter-wave plate 121 twice, thereby the electric field direction of the light beam symmetrically reversed on a surface including an optical axis 100 in the drawing and being vertical to the paper surface. As a result, the electric field direction of the light 200S reflected on the polarization selecting surface 101A (represented by a solid line) on the incident side as a S-polarized component coincides with the electric field direction of a light beam reflected on a virtual polarization selecting surface 101B (represented by a dotted line) as S-polarized light. The direction coincides with the S-polarized component on the PBS 101 on the emission side, so the direction is well removed in the PBS 101 on the emission side, thereby degradation of the extinction ratio (=incident light/emission light) can be prevented. For example, a conventional technique using such a quarter-wave plate is proposed in Japanese Unexamined Patent Application Publication No. Hei 10-26756.
A correction technique using the quarter-wave plate effectively works, when an ideal retardation plate which produces a phase difference of ¼ wavelength for any light beam is used. However, in reality, the amount of phase difference varies depending upon an incident angle, so when a light beam with a large angle with respect to an optical axis is included, degradation of the extinction ratio occurs. There is a tendency that the larger the incident angle is, or the thicker the retardation plate is, the more the extinction ratio is reduced.
Referring to FIGS. 21A through 21C and 22A through 22C, a commonly used retardation plate will be described below. When light enters a crystal with optical anisotropy such as quartz crystal, a refractive index is different depending upon an electric field direction, so a difference in wavelength occurs, thereby a phase difference corresponding to a difference in wave number occurs. In a retardation plate 130 shown in FIG. 21A, an ordinary ray (a light beam having a refractive index no) is higher in speed than an extraordinary ray (a light beam having a refractive index ne), thereby when the wavelength increases, a phase difference occurs. FIGS. 21B and 21C schematically show a state of the extraordinary ray and a state of the ordinary ray in the retardation plate 130, respectively.
The retardation plate introduces a phase difference between orthogonal components of incident light, and in the retardation plate, between two vibration components orthogonal to each other, a vibration direction of a vibration component with a higher phase speed is called “fast axis” and a vibration direction of a vibration component with a lower phase speed is called “slow axis”. In FIGS. 21A through 21C, a direction of the refractive index no of the ordinary ray (x-direction) is a fast axis, and a direction of the refractive index ne of the extraordinary ray (y-direction) is a slow axis.
In the quartz crystal, an optical path length (thickness) required to produce a phase difference of ¼ wavelength between the ordinary ray and the extraordinary ray is approximately 15 microns. It is difficult to actually form a retardation plate of quartz crystal with this thickness, because the thickness is too thin, so as shown in FIG. 22A, in general, a first retardation plate 131 and a second retardation plate 132 which each produce a different phase difference are combined so that the total phase difference produced by a combination of the retardation plates 131 and 132 is adjusted to be ¼ wavelength. In this case, the retardation plates 131 and 132 are placed so that a positional relationship between the axis of the refractive index no (fast axis) and the axis of the refractive index ne (slow axis) are 90° different from each other.
FIGS. 22B and 22C schematically show a state that incident light beams 141 and 142 of which the vibration directions are orthogonal to each other pass through the retardation plates 131 and 132. The light beam 141 is a component of which the vibration direction is oriented in a y-direction in FIG. 22A, and the light beam 142 is a component of which the vibration direction is oriented in an x-direction in FIG. 22A. The incident light beam 142 is converted into an ordinary ray on the first retardation plate 131 so that the phase of the light beam 142 is (M+¼)λ leading, and then the light beam 142 is converted into an extraordinary ray on the second retardation plate 132 so that the phase of the light beam 142 is Mλ lagging, thereby the light beam 142 has the total phase difference of ¼ wavelength with respect to the incident light beam 141. Incidentally, λ represents one wavelength. Conventionally, when a phase difference produced by each of the retardation plates 131 and 132 is determined, mainly only manufacturability (thickness) is considered. Therefore, as long as the retardation plates can produce a phase difference of ¼ wavelength in total, specific structures of the retardation plates present no problem. For example, in the case where retardation plates are made of quartz crystal, two retardation plates have a total thickness of 600 microns or over in general.
Thus, in the case where two retardation plates are combined, as a result, when an eventual incident angle is small (close to vertical incidence), the retardation plates function properly; however, fluctuations in phase difference with respect to a diagonally incident light beam increase, thereby degradation of the extinction ratio occurs. When an incident angle is limited within a small range in order to prevent the degradation, another problem that a decline in the amount of available light results in a darker image area arises.
As another material of the retardation plate, there is an organic film having optical anisotropy by drawing or the like. In the case of the organic film, compared to quartz crystal, a difference between the refractive index no of the ordinary ray and the refractive index ne of the extraordinary ray is smaller, so depending upon materials, the organic film with a thickness of approximately 60 microns can produce a phase difference of ¼ wavelength. When the organic film can have such a thickness, two organic films are not required, and the retardation plate can be formed with only one organic film, thereby a thin retardation plate can be achieved. The degradation of the extinction ratio is caused by the thickness of the retardation plate and a difference between the refractive index of the ordinary ray and the refractive index of the extraordinary ray, so the performance of the organic film with a thickness of approximately 60 microns is as good as that of quartz crystal with a thickness of approximately 15 microns. Although the organic film has been already used in projectors and the like as a quarter-wave plate, the organic film is vulnerable to a temperature rise, so a problem of long-term reliability arises.
In summary, when an ideal quarter-wave plate is placed between the reflective liquid crystal display panel 102 and the PBS 101, the polarization state can be corrected properly. However, in a practical quarter-wave plate, a phase difference for a light beam diagonally passing therethrough varies depending upon conditions of the entry, so the polarization state is not sufficiently corrected, thereby the following problems of image quality arise in the projector.
1) As light of a component of which the polarization state cannot be corrected enters an image area, an area which is supposed to be dark is not sufficiently dark.
2) When diagonally incident light is limited in order to avoid the above problem, the amount of available light is reduced, thereby the whole image area becomes dark.
A typically used organic material film is formed so as to have a minimum thickness required to produce a phase difference of approximately ¼ wavelength, thereby the above decline in image quality is avoided as much as possible; however, the organic material is vulnerable to a temperature rise, thereby a problem of long-term reliability arises. On the other hand, quartz crystal which is a more typical material of the wave plate is resistant to a temperature rise, and has superior long-term reliability; however, there is a problem that it is difficult to form the wave plate with a minimum thickness required to produce a phase difference of approximately ¼ wavelength. A typical quartz crystal wave plate includes a combination of two wave plates, thereby the quartz crystal wave plate produces a phase difference of approximately ¼ wavelength, so compared to the organic material film, an effective thickness of the quartz crystal wave plate increases, thereby the quartz crystal wave plate has the above two problems that a decline in image quality becomes pronounced.
In view of the foregoing, it is a first object of the invention to provide a projector capable of properly correcting a polarization state to improve image quality. Moreover, it is a second object of the invention to provide a retardation plate being used in a projector or the like and being capable of properly correct a polarization state and a method of placing a retardation plate.